99-517: Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design. The prime objective of a calendar is to unambiguously identify any day in past, present and future by a specific date in order to record or organize social, religious, commercial or administrative events. Recurring periods that contain multiple days, such as weeks , months , and years , are secondary, convenient features of
198-649: A better fit to the sidereal year. There have been reforms of the solar version of the Hindu calendar which changed the distribution of the days in each month to better match the length of time that the Sun spends in each sidereal zodiacal sign. The same applies to the Buddhist calendar . The first millennium reform of the Hebrew calendar changed it from an observational calendar into a calculated calendar. The Islamic calendar
297-593: A business. It is used for budgeting, keeping accounts, and taxation. It is a set of 12 months that may start at any date in a year. The US government's fiscal year starts on 1 October and ends on 30 September. The government of India's fiscal year starts on 1 April and ends on 31 March. Small traditional businesses in India start the fiscal year on Diwali festival and end the day before the next year's Diwali festival. Hindu calendar Traditional The Hindu calendar , also called Panchanga ( Sanskrit : पञ्चाङ्ग ),
396-559: A calendar based on the reign of their current sovereign. For example, the year 2006 in Japan is year 18 Heisei, with Heisei being the era name of Emperor Akihito . An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple . Such a calendar is also referred to as an observation-based calendar. The advantage of such
495-407: A calendar includes more than one type of cycle or has both cyclic and non-cyclic elements. Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia. Solar calendars assign a date to each solar day . A day may consist of
594-437: A calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult. An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar . Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage
693-435: A calendar provides a way to determine when to start planting or harvesting, which days are religious or civil holidays , which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also, a calendar may, by identifying a day, provide other useful information about the day such as its season. Calendars are also used as part of
792-421: A calendar. Most cultures adopt a primary dating system, but different cultures have always needed to align multiple calendars with each other, either because they coexisted in the same space (e.g. secular and religious groups with different demands) or had established trading relations. Once specified, a calendar design cannot be altered without becoming a new design. If a proposed design is sufficiently close to
891-417: A complete timekeeping system: date and time of day together specify a moment in time . In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase. The Gregorian calendar is the de facto international standard and is used almost everywhere in the world for civil purposes. The widely used solar aspect is a cycle of leap days in a 400-year cycle designed to keep
990-411: A device to track solar day-to-day progression, such as that established at places like Stonehenge . After centuries of empirical observations, the theoretical aspects of calendar construction could become more refined, enabling predictions that identified the need for reform. There have been 50 to 100 reforms of the traditional Chinese calendar over 2500 years, most of which were intended to better fit
1089-430: A leap month called Meton every 3 or 2 years with 30 or 31 days. 60 years together are called a cycle. It uses a leap cycle which has equal number of days, weeks, months, years and cycles. 2498258 days, 356894 weeks, 84599 months, 6840 years and 114 cycles nearly all equal each other. It is called an era, although time isn't divided into it in this calendar. Some propose to improve leap rules of existing calendars, such as
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#17327663072051188-482: A leap week of seven days to the calendar every five or six years to keep the calendar roughly in step with the tropical year. They have years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week. Proposals mainly differ in their selection of a leap rule, placing of the leap item (usually middle or end of the year), in the start day of the week and year, in the number (12 or 13) and size of months and in connected naming; some are compatible to
1287-557: A lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation . Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length. Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars on regnal years ,
1386-470: A lunar system. The Buddhist calendar and the traditional lunisolar calendars of Cambodia , Laos , Myanmar , Sri Lanka and Thailand are also based on an older version of the Hindu calendar. Similarly, the ancient Jain traditions have followed the same lunisolar system as the Hindu calendar for festivals, texts and inscriptions. However, the Buddhist and Jain timekeeping systems have attempted to use
1485-457: A number of systems of which intercalary months became most used, that is adding another month every 32.5 months on average. As their calendar keeping and astronomical observations became more sophisticated, the Hindu calendar became more sophisticated with complex rules and greater accuracy. According to Scott Montgomery, the Siddhanta tradition at the foundation of Hindu calendars predate
1584-563: A reference date. This applies for the Julian day or Unix Time . Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction. Other calendars have one (or multiple) larger units of time. Calendars that contain one level of cycles: Calendars with two levels of cycles: Cycles can be synchronized with periodic phenomena: Very commonly
1683-597: A rudimentary level. Later medieval era texts such as the Yavana-jataka and the Siddhanta texts are more astrology-related. Hinduism and Buddhism were the prominent religions of southeast Asia in the 1st millennium CE, prior to the Islamic conquest that started in the 14th century. The Hindus prevailed in Bali, Indonesia, and they have two types of Hindu calendar. One is a 210-day based Pawukon calendar which likely
1782-577: A sabbath every seven days. Independently the World Council of Churches still tries to find a common rule for the date of Easter, which might be eased by a new common calendar. Reformers cite several problems with the Gregorian calendar: It is hard or even impossible to solve all these issues in just one calendar. Most plans evolve around the solar year of a little more than 365 days. This number does not divide well by seven or twelve, which are
1881-750: A sermon given on 9 Dhu al-Hijjah AH 10 (Julian date: 6 March 632). This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year. There have been several modern proposals for reform of the modern calendar, such as the World Calendar , the International Fixed Calendar , the Holocene calendar , and the Hanke–Henry Permanent Calendar . Such ideas are mooted from time to time, but have failed to gain traction because of
1980-525: A similar manner to the Christian era . There are several samvat found in historic Buddhist, Hindu and Jain texts and epigraphy, of which three are most significant: Vikrama era, Old Shaka era and Shaka era of 78 CE. The Hindu calendar divides the zodiac into twelve division called rāśi ("group"). The Sun appears to move around the Earth through different divisions/constellations in the sky throughout
2079-580: A solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star— Sirius , or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate. Not all calendars use
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#17327663072052178-479: A whole number of days nor a whole number of lunar months; neither does the lunar month have a whole number of days: in each case there are fractions "left over". (In some physical circumstances the rotations and revolutions of a planet and its satellite(s) can be phase-locked – for example the same side of the moon always faces us – but this has not operated to lock together the lengths of the Earth's year, day and month.) Such remainders could accumulate from one period to
2277-518: Is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar) and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the Hijra (corresponding to AD 622). With an annual drift of 11 or 12 days, the seasonal relation
2376-597: Is a pre-Hindu system, and another is similar to lunisolar calendar system found in South India and it is called the Balinese saka calendar which uses Hindu methodology. The names of month and festivals of Balinese Hindus, for the most part, are different, though the significance and legends have some overlap. The Hindu calendar is based on a geocentric model of the Solar System . A large part of this calendar
2475-399: Is a variant of this concept. Each year of this calendar can be up to 371 days long. Some calendars have quarters of regularly patterned uneven months e.g., a 35-day (five-week) month and a pair of 28-day (four-week) months, with a leap week appended to the final month when needed. The Common Civil Calendar and Time calendar has months of 30 and 31 days, but inserts a leap week in the middle of
2574-563: Is also found in the Hebrew calendar , the Chinese calendar , and the Babylonian calendar , but different from the Gregorian calendar. Unlike the Gregorian calendar which adds additional days to the month to adjust for the mismatch between twelve lunar cycles (354 lunar days) and approximately 365 solar days, the Hindu calendar maintains the integrity of the lunar month, but inserts an extra full month, once every 32–33 months, to ensure that
2673-552: Is defined based on the movement of the Sun and the Moon around the Earth (saura māna and cāndra māna respectively). Furthermore, it includes synodic , sidereal , and tropical elements. Many variants of the Hindu calendar have been created by including and excluding these elements (solar, lunar, lunisolar etc.) and are in use in different parts of India. Samvat refers to era of the several Hindu calendar systems in Nepal and India , in
2772-423: Is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar. The early Roman calendar , created during the reign of Romulus , lumped
2871-451: Is one of various lunisolar calendars that are traditionally used in the Indian subcontinent and Southeast Asia , with further regional variations for social and Hindu religious purposes. They adopt a similar underlying concept for timekeeping based on sidereal year for solar cycle and adjustment of lunar cycles in every three years, but differ in their relative emphasis to moon cycle or
2970-691: Is repeated approximately every 33 Islamic years. Various Hindu calendars remain in use in the Indian subcontinent, including the Nepali calendars , Bengali calendar , Malayalam calendar , Tamil calendar , Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan states. The Buddhist calendar and the traditional lunisolar calendars of Cambodia , Laos , Myanmar , Sri Lanka and Thailand are also based on an older version of
3069-501: Is the Islamic calendar . Alexander Marshack, in a controversial reading, believed that marks on a bone baton ( c. 25,000 BC ) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar. A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign
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3168-406: Is the designation of a single and specific day within such a system. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as a court calendar , or a partly or fully chronological list of documents, such as a calendar of wills. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with
3267-433: Is the large irregularity of the length of the year from one to the next. The simplicity of a lunar calendar has always been outweighed by its inability to track the seasons, and a solar calendar is used in conjunction to remedy this defect. Identifying the lunar cycle month requires straightforward observation of the Moon on a clear night. However, identifying seasonal cycles requires much more methodical observation of stars or
3366-439: Is used for each season of every year. A decimal calendar is a calendar which includes units of time based on the decimal system . The French Republican Calendar was introduced (along with decimal time ) in 1793. It consisted of twelve months, each divided into three décades of ten days, with five or six intercalary days called sansculottides . The calendar was abolished by Napoleon on January 1, 1806. The lengths of
3465-632: Is used in Iran and some parts of Afghanistan . The Assyrian calendar is in use by the members of the Assyrian community in the Middle East (mainly Iraq, Syria, Turkey, and Iran) and the diaspora. The first year of the calendar is exactly 4750 years prior to the start of the Gregorian calendar. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea , with
3564-747: The Babylonian calendar dating from the Iron Age , among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar . A great number of Hellenic calendars were developed in Classical Greece , and during the Hellenistic period they gave rise to the ancient Roman calendar and to various Hindu calendars . Calendars in antiquity were lunisolar , depending on
3663-720: The Bronze Age Egyptian and Sumerian calendars. During the Vedic period India developed a sophisticated timekeeping methodology and calendars for Vedic rituals. According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures. A large number of calendar systems in the Ancient Near East were based on
3762-572: The Chinese language in the 2nd and 3rd centuries CE, and the Rigvedic passages on astronomy are found in the works of Zhu Jiangyan and Zhi Qian . According to Subhash Kak , the beginning of the Hindu calendar was much earlier. He cites Greek historians describing Maurya kings referring to a calendar which originated in 6676 BCE known as Saptarsi calendar. The Vikrami calendar is named after king Vikramaditya and starts in 57 BCE. Hindu scholars kept precise time by observing and calculating
3861-409: The Hebrew calendar . The Rectified Hebrew calendar uses a more accurate leap cycle of 4366 months per 353-year cycle, with 130 leap years per cycle, and a progressively shorter molad interval, intended to replace the 19-year leap cycle and the constant molad interval of the traditional fixed arithmetic Hebrew calendar, respectively. Calendar proposals that introduce a thirteenth month or change
3960-566: The Oromo calendar also in use in some areas. In neighboring Somalia , the Somali calendar co-exists alongside the Gregorian and Islamic calendars. In Thailand , where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar . A fiscal calendar generally means the accounting year of a government or
4059-518: The Pax Calendar , which avoids off-calendar days by adding a 7-day leap week to the 364-day common year for 71 out of 400 years. Lunisolar calendars usually have 12 or 13 months of 29 or 30 days. The Hermetic Lunar Week Calendar is a lunisolar calendar proposal which has 12 or 13 lunar months of 29 or 30 days a year, and begins each year near the vernal equinox . The Meyer–Palmen Solilunar Calendar has 12 lunar months with 29 or 30 days plus
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4158-686: The week date of ISO 8601. The World Calendar , favored by the UN in the 1950s, and the International Fixed Calendar , quite popular among economists between the World Wars, are proposals that start each year on a Sunday. The 364 days within the week system form 52 weeks of 7 days. The World Calendar has every quarter beginning on the same day of the week. In the World Calendar, the 365th and 366th day are considered holidays and named Worlds Day and Leap Year Day. These "off-calendar" days stand outside
4257-522: The "calling" of the new moon when it was first seen. Latin calendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century (the spelling calendar is early modern). The course of the Sun and
4356-530: The 61 days of the winter period them together as simply "winter." Over time, this period became January and February; through further changes over time (including the creation of the Julian calendar ) this calendar became the modern Gregorian calendar, introduced in the 1570s. The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example,
4455-570: The Buddha and the Mahavira's lifetimes as their reference points. The Hindu calendar is also important to the practice of Hindu astrology and zodiac system. It is also employed for observing the auspicious days of deities and occasions of fasting, such as Ekadashi . Time keeping [The current year] minus one, multiplied by twelve, multiplied by two, added to the elapsed [half months of current year], increased by two for every sixty [in
4554-603: The Christian era, once had 18 texts of which only 5 have survived into the modern era. These texts provide specific information and formulae on motions of Sun, Moon and planets, to predict their future relative positions, equinoxes, rise and set, with corrections for prograde, retrograde motions, as well as parallax. These ancient scholars attempted to calculate their time to the accuracy of a truti (29.63 microseconds). In their pursuit of accurate tracking of relative movements of celestial bodies for their calendar, they had computed
4653-422: The Gregorian calendar: The following count one or more days outside the standard seven-day week: The following are leap week calendars: There have also been proposals to revise the way years are numbered: Reform of the Islamic calendar: Calendar A calendar is a system of organizing days . This is done by giving names to periods of time , typically days, weeks , months and years . A date
4752-687: The Hindu calendar. Most of the Hindu calendars are inherited from a system first enunciated in Vedanga Jyotisha of Lagadha, standardized in the Sūrya Siddhānta and subsequently reformed by astronomers such as Āryabhaṭa (AD 499), Varāhamihira (6th century) and Bhāskara II (12th century). The Hebrew calendar is used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays ) and can be used business dealings (such as for
4851-613: The Julian calendar, although the Russian church year still uses the Julian calendar. Those nations that adopted this calendar on or after 1700, had to drop more than ten days: Great Britain, for instance, dropped eleven. In 1923, Milutin Milanković proposed to a synod of some Eastern Orthodox Churches at Constantinople that only centennial years that leave a remainder of 200 or 600 upon division by 900 would be leap years, decreasing
4950-552: The Julian-Gregorian system of months often also propose new names for these months. New names have also been proposed for days out of the week cycle (e.g., 365th and leap) and weeks out of the month cycle. Proposals to change the traditional month and weekday names are less frequent. The Gregorian calendar obtains its names mostly from gods of historical religions (e.g., Thursday from Nordic Thor or March from Roman Mars ) or leaders of vanished empires (July and August from
5049-626: The Malayalam calendar broadly retains the phonetic Sanskrit names, the Bengali and Tamil calendars repurpose the Sanskrit lunar month names (Chaitra, Vaishaka etc.) as follows: The solar months ( rāśi ) along with their equivalent names in the Bangali, Malayalam and Tamil calendar are given below: or ଭାଦ୍ର (Bhādra) (Tai) or ଫଗୁଣ (Phaguṇa) (Māsi) The solar months ( rāśi ) along with
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#17327663072055148-539: The Moon are the most salient regularly recurring natural events useful for timekeeping , and in pre-modern societies around the world lunation and the year were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year. The first recorded physical calendars, dependent on the development of writing in the Ancient Near East , are
5247-604: The Old Calendar) and the Revised Julian Calendar (often called the New Calendar). The Revised Julian Calendar is nearly the same as the Gregorian calendar, with the addition that years divisible by 100 are not leap years , except that years with remainders of 200 or 600 when divided by 900 remain leap years, e.g. 2000 and 2400 as in the Gregorian calendar. The Islamic calendar or Hijri calendar
5346-546: The Roman Rite of the Catholic Church, and generally include the liturgical seasons of Advent , Christmas , Ordinary Time (Time after Epiphany ), Lent , Easter , and Ordinary Time (Time after Pentecost ). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season. The Eastern Orthodox Church employs the use of 2 liturgical calendars; the Julian calendar (often called
5445-711: The Sun towards north for 6 months, and south for 6 months. Time keeping was important to Vedic rituals, and Jyotisha was the Vedic era field of tracking and predicting the movements of astronomical bodies in order to keep time, in order to fix the day and time of these rituals. This study is one of the six ancient Vedangas , or ancillary science connected with the Vedas – the scriptures of Vedic Sanatan Sanskriti. Yukio Ohashi states that this Vedanga field developed from actual astronomical studies in ancient Vedic Period. The texts of Vedic Jyotisha sciences were translated into
5544-424: The average year length to 365.24 2 days: these remainders were chosen to delay as much as possible the first year (after the year of 1923) that this calendar would disagree with the Gregorian calendar, which occurs in until 2800. It was adopted by some Eastern Orthodox Churches, under the names Revised Julian calendar or New calendar, but was rejected by others. The Gregorian calendar is currently used by most of
5643-457: The calendar months to astronomical lunations and to more accurately add the extra month so that the regular months maintain their proper seasonal positions, even though each seasonal marker can occur anywhere within its month. There have been at least four similar reforms of the lunisolar version of the Hindu calendar , all intended to make the month a better match to the lunation and to make the year
5742-482: The calendar with the astronomical year (either solar or sidereal ) and/or the synodic month in lunar or lunisolar calendars . Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar , when it was altered to the Gregorian calendar . The fundamental problems of the calendar are that the astronomical year has neither
5841-447: The characteristics of the respective planetary motion. Other texts such as Surya Siddhanta dated to have been completed sometime between the 5th century and 10th century present their chapters on various deified planets with stories behind them. The manuscripts of these texts exist in slightly different versions. They present Surya, planet-based calculations and Surya's relative motion to Earth. These vary in their data, suggesting that
5940-421: The cycle of the sun or the moon . The most common type of pre-modern calendar was the lunisolar calendar , a lunar calendar that occasionally adds one intercalary month to remain synchronized with the solar year over the long term. The term calendar is taken from kalendae , the term for the first day of the month in the Roman calendar, related to the verb calare 'to call out', referring to
6039-531: The cycles of Surya (the Sun), Moon and the planets. These calculations about the Sun appear in various astronomical texts in Sanskrit , such as the 5th-century Aryabhatiya by Aryabhata , the 6th-century Romaka by Latadeva and Panca Siddhantika by Varahamihira, the 7th-century Khandakhadyaka by Brahmagupta and the 8th-century Sisyadhivrddida by Lalla. These texts present Surya and various planets and estimate
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#17327663072056138-716: The dating of cheques ). Followers of the Baháʼí Faith use the Baháʼí calendar . The Baháʼí Calendar, also known as the Badi Calendar was first established by the Bab in the Kitab-i-Asma. The Baháʼí Calendar is also purely a solar calendar and comprises 19 months each having nineteen days. The Chinese , Hebrew , Hindu , and Julian calendars are widely used for religious and social purposes. The Iranian (Persian) calendar
6237-418: The duration of the year aligned with the solar year . There is a lunar aspect which approximates the position of the moon during the year, and is used in the calculation of the date of Easter . Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days). The Gregorian calendar
6336-493: The festivals and crop-related rituals fall in the appropriate season. The Hindu calendars have been in use in the Indian subcontinent since Vedic times, and remain in use by the Hindus all over the world, particularly to set Hindu festival dates. Early Buddhist communities of India adopted the ancient Vedic calendar,later Vikrami calendar and then local Buddhist calendars . Buddhist festivals continue to be scheduled according to
6435-606: The first Caesars), or ordinals that got out of synchronization (September through December, originally seventh through tenth, now ninth through twelfth). Comte's Positivist calendar, for example, proposed naming the 13 months in his calendar after figures from religion, literature, philosophy and science. Similarly, the Hermetic Lunar Week Calendar uses 12 or 13 lunar months named after 13 contributors to research on psychoactive plants and chemicals. There have been many specific calendar proposals to replace
6534-457: The intercalary month with an intercalary day to be inserted within February every four years. This produced a noticeably more accurate calendar, but it had an average year length of 365 days and six hours (365.25 days), which had the effect of adding about three-quarters of an hour every four years. The effect accumulated from inception in 45 BC until by the 16th century the northward equinox
6633-415: The introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar . The Roman calendar was reformed by Julius Caesar in 46 BC. His "Julian" calendar was no longer dependent on the observation of
6732-533: The legacy one, i.e. compatible with it, a reform of the local calendar system is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar , where the start of the month is now derived from astronomical data rather than sightings by religious leaders. Some design changes, however, will yield date identifiers different from
6831-432: The loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity. A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years. The simplest calendar system just counts time periods from
6930-669: The lunar cycle. Their new year starts in spring. In regions such as Tamil Nadu and Kerala, the solar cycle is emphasized and this is called the Tamil calendar (though Tamil Calendar uses month names like in Hindu Calendar) and Malayalam calendar and these have origins in the second half of the 1st millennium CE. A Hindu calendar is sometimes referred to as Panchangam (पञ्चाङ्गम्), which is also known as Panjika in Eastern India. The ancient Hindu calendar conceptual design
7029-466: The mean diameter of the Earth, which was very close to the actual 12,742 km (7,918 mi). Hindu calendars were refined during the Gupta era astronomy by Āryabhaṭa and Varāhamihira in the 5th to 6th century. These, in turn, were based in the astronomical tradition of Vedāṅga Jyotiṣa , which in the preceding centuries had been standardised in a number of (non-extant) works known as Sūrya Siddhānta . Regional diversification took place in
7128-651: The medieval period. The astronomical foundations were further developed in the medieval period, notably by Bhāskara II (12th century). Later, the term Jyotisha evolved to include Hindu astrology . The astrological application of the Hindu calendar was a field that likely developed in the centuries after the arrival of Greek astrology with Alexander the Great , because their zodiac signs are nearly identical. The ancient Hindu texts on Jyotisha only discuss timekeeping, and never mention astrology or prophecy. These ancient texts predominantly cover astronomy, but at
7227-509: The months inherited from the old Roman calendar as reformed by Julius Caesar do not follow any apparent logic systematically. Many reform proposals seek to make the pattern more uniform. When keeping the traditional dozen months and the close approximation of a solar year, this usually yields four equal quarters of three months each where one month is longer than the other two. World Calendar and Hanke–Henry Permanent Calendar follow this with 31:30:30 and 30:30:31 days per month, respectively. On
7326-516: The months with the seasons. Prominent examples of lunisolar calendar are Hindu calendar and Buddhist calendar that are popular in South Asia and Southeast Asia . Another example is the Hebrew calendar, which uses a 19-year cycle . Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is,
7425-416: The new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from lunation . The Gregorian calendar , introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year. The Islamic calendar is based on the prohibition of intercalation ( nasi' ) by Muhammad , in Islamic tradition dated to
7524-418: The next, thereby driving the cycles out of synchronization. A typical solution to force synchronization is intercalation . This means occasionally adding an extra day, week, or month into the cycle. An alternative approach is to ignore the mismatch and simply let the cycles continue to drift apart. The general approaches include: An obvious disadvantage of the lunisolar method of inserting a whole extra month
7623-454: The observation of religious feast days. While the Gregorian calendar is itself historically motivated to the calculation of the Easter date , it is now in worldwide secular use as the de facto standard. Alongside the use of the Gregorian calendar for secular matters, there remain several calendars in use for religious purposes. Western Christian liturgical calendars are based on the cycle of
7722-419: The other hand, Symmetry454 uses 4:5:4 weeks per month. They all result in 364 systematically distributed days and hence have to add either one extra and one leap day or a leap week. Some calendar reformers seek to equalize the length of each month in the year. This is often accomplished by creating a calendar that has 13 months of 4 weeks (28 days) each, making 364 days. The earliest known proposal of this type
7821-414: The period between sunrise and sunset , with a following period of night , or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day . Other types of calendar may also use a solar day. The Egyptians appear to have been the first to develop
7920-626: The previous design for some days, often in the distant past or future. The calendar system must clarify whether dates are changed to the new design retroactively (using a proleptic calendar ) or whether the design in use then and there shall be respected. Calendar schisms happen if not all cultures that adopted a common calendar system before perform a calendar reform at the same time. If date identifiers are similar but different, confusion and mistakes are unavoidable. Most calendars have several rules which could be altered by reform: Historically, most calendar reforms have been made in order to synchronize
8019-408: The same challenge of accounting for the mismatch between the nearly 354 lunar days in twelve months, versus over 365 solar days in a year. They tracked the solar year by observing the entrance and departure of Surya (sun, at sunrise and sunset) in the constellation formed by stars in the sky, which they divided into 12 intervals of 30 degrees each. Like other ancient human cultures, Hindus innovated
8118-399: The seven-day week and caused some religious groups to strongly oppose adoption of the World Calendar. Such concerns helped prevent the World Calendar from being adopted. Supporters of the World Calendar, however, argue that the religious groups' opposition overlooked every individual's right to celebrate these holidays as extra days of worship, or Sabbaths . This option, they reason, maintained
8217-467: The seven-day worship cycle for those who share that concern, while allowing benefits of a perennial calendar to be shared by all. Some calendar reform ideas, such as the Pax Calendar , Symmetry454 calendar and the Hanke–Henry Permanent Calendar , were created to solve this problem by having years of either 364 days (52 weeks) or 371 days (53 weeks), thus preserving the 7-day week. The 53-week calendar, used in government and in business for fiscal years ,
8316-407: The solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year , a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides . An example
8415-783: The sun cycle and the names of months and when they consider the New Year to start. Of the various regional calendars, the most studied and known Hindu calendars are the Shalivahana Shaka (Based on the King Shalivahana , also the Indian national calendar ) found in the Deccan region of Southern India and the Vikram Samvat (Bikrami) found in Nepal and the North and Central regions of India – both of which emphasize
8514-417: The sun], is the quantity of half-months ( syzygies ). — Rigveda Jyotisha-vedanga 4 Translator: Kim Plofker The Vedic culture developed a sophisticated time keeping methodology and calendars for Vedic rituals, and timekeeping as well as the nature of solar and Moon movements are mentioned in Vedic texts. For example, Kaushitaki Brahmana chapter 19.3 mentions the shift in the relative location of
8613-468: The text were open and revised over their lives. For example, the 1st millennium CE Hindu scholars calculated the sidereal length of a year as follows, from their astronomical studies, with slightly different results: The Hindu texts used the lunar cycle for setting months and days, but the solar cycle to set the complete year. This system is similar to the Jewish and Babylonian ancient calendars, creating
8712-528: The time it takes for a complete cycle of seasons ), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar , the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week. Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in leap years . The same applies to months in
8811-548: The traditional numbers of days per week and months per year respectively. The nearby numbers 360, 364 and 366 are divisible in better ways. There are also lunar-centric proposals. Many calendar reforms have offered solutions to make the Gregorian calendar perennial. These reforms would make it easy to work out the day of the week of a particular date , and would make changing calendars each year unnecessary. There are, roughly speaking, two options to achieve this goal: leap week calendars and intercalary days . Leap week calendars add
8910-425: The world. There is also an international standard describing the calendar, ISO 8601 , with some differences from traditional conceptions in many cultures. Since the papal reform in 1582, several proposals have been offered to make the Gregorian calendar more useful or regular. Very few reforms have gained official acceptance. The rather different decimal French Republican Calendar was one such official reform, but
9009-576: The year, when needed, whereas its successor, the Hanke–Henry Permanent Calendar, moves the extra week to the end of the year. In the World Season Calendar , months are discarded altogether; instead, the year is divided into four seasons of 13 weeks each. An extra day (two days during leap year) is added to the calendar that is not assigned a day of the week in order to perennialize the calendar. The same calendar of 91 days
9108-439: The year, which in reality is actually caused by the Earth revolving around the Sun. The rāśi s have 30° each and are named for constellations found in the zodiac. The time taken by the Sun to transit through a rāśi is a solar month whose name is identical to the name of the rāśi. In practice, solar months are mostly referred as rāśi (not months). The solar months are named differently in different regional calendars. While
9207-510: The years entirely, it would require a few thousand years to accumulate a day. So that the northward equinox would have the same date in the new Gregorian calendar as it had when the Council of Nicaea made recommendations in AD ;325 ( March 21 ), ten days were dropped so that October 5 became October 15 in 1582. This reform took a few centuries to spread through the nations that used
9306-399: Was Greece, in 1923. The calendar epoch used by the Gregorian calendar is inherited from the medieval convention established by Dionysius Exiguus and associated with the Julian calendar. The year number is variously given as AD (for Anno Domini ) or CE (for Common Era or Christian Era ). The most important use of pre-modern calendars is keeping track of the liturgical year and
9405-567: Was a reform of the preceding lunisolar calendar which completely divorced it from the solar year. Another reform was performed in Seljuk Persia by Omar Khayyam and others, developing the precisely computed Jalali calendar . When Julius Caesar took power in Rome, the Roman calendar had ceased to reflect the year accurately. The Julian reform made 46 BC 445 days long and replaced
9504-570: Was abolished twelve years later by Napoleon . After World War II , the newly formed United Nations continued efforts of its predecessor, the League of Nations , to establish the proposed World Calendar but postponed the issue after a veto from the government of the United States , which was mainly based upon concerns of religious groups about the proposed days that would be outside the seven-day week cycle ("blank days") and thus disrupt having
9603-470: Was falling on March 10 or 11. Under Pope Gregory XIII , the leap year rule was altered : only centennial years evenly divisible by 400 are leap years. Thus, the years 1600, 2000, 2400 and 2800 are leap years , while 1700, 1800, 1900, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000 are common years despite being divisible by 4. This rule makes the mean year 365.2425 days (365 d, 5 h, 49 min, 12 s) long. While this does not synchronize
9702-554: Was introduced in 1582 as a refinement to the Julian calendar , that had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year. During the Early Modern period, its adoption was mostly limited to Roman Catholic nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it
9801-543: Was the Georgian Calendar (1745) by Rev. Hugh Jones . The Positivist calendar (1849), created by Auguste Comte , was based on a 364-day year which included one or two "blank" days. Each of the 13 months had 28 days and exactly four weeks, and each started on a Monday. The International Fixed Calendar is a more modern descendant of this calendar: invented by Moses B. Cotsworth and financially backed by George Eastman . Around 1930, one James Colligan invented
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